High Proton Conductivity Enhancement Obtained by a Covalent Postsynthesis Modification Approach for Two Metal–Organic Frameworks

This study demonstrates an effective strategy to enhance proton conductivity by synthesizing 2 three-dimensional metal-organic frameworks (MOFs), [Zn(DTD22)]_<i>n</i> (MOF 1) and [Cd₂(DTD22)₂]_<i>n</i> (MOF 2), (DTD22 = 4,4″-diamino-[1,1':4',1″-terphenyl]-2,2″-dicarboxylic acid). The DTD22 ligand used formed a continuous hydrogen-bonding network in the structure, constructing excellent hydrophilic channels. MOF 1 and MOF 2 were further postsynthesized and modified (PSM) by Schiff base reaction, and 4-chloro-3-formylbenzenesulfonic acid ligands containing -SO₃H and -Cl were successfully introduced into the framework to form PSM-MOF 1 and PSM-MOF 2. Experiments showed that this modification significantly enhanced the proton conductivity of the materials, especially at 90 °C and 98% RH: PSM-MOF 1 (2.38 × 10^-1 S·cm^-1) and PSM-MOF 2 (3.50 × 10^-1 S·cm^-1). In comparison, the conductivities of unmodified MOF 1 and MOF 2 were 8.55 × 10^-2 S·cm^-1 and 9.50 × 10^-5 S·cm^-1, respectively. The present study demonstrates that the proton conductivity of MOFs can be effectively enhanced by the covalent postmodification method, which provides a new idea for the application of MOFs.